1. Field of the Invention
The present invention relates to methods and systems for the planning and delivery of radiation therapy. In particular, the invention relates to methods and systems for real-time tumor tracking with a preprogrammed delivery sequence and adaptive treatment speed modulation to account for tumor motion caused by breathing.
2. Description of Related Art
Radiation therapy is used to treat cancers and other conditions in patients. One commonly used form of radiation therapy is external beam radiation therapy. In external beam radiation therapy, a high energy x-ray beam generated by a machine (usually a linear accelerator) or a charged particle beam, generated by a particle accelerator, (herein, an accelerator means either an x-ray linear accelerator or a particle accelerator for description convenience) located outside of the patient's body is directed at a tumor or cancerous cells inside the patient's body. While the radiation kills the cancerous cells it also harms normal tissue and organs in the vicinity of the tumor/cancerous cells in the patient. Thus, a goal in radiation therapy is to deliver the required dose of radiation to the smallest possible target volume to minimize the radiation dose that may impact surrounding normal tissue.
There are several sources of error that are encountered in radiation therapy. First, there is error involved in delineating the boundaries of the target (as used herein, “target” refers to the area in which the delivery of radiation is desired such as a tumor). Second, there is error due to target motion. Target motion refers to motion of a target due to patient bodily functions, during irradiation and between treatment sessions. For example, breathing will cause motion in the organs located in the thorax and abdomen of a patient, during the treatment. This motion can take the form of translation of the target (e.g., a patient's liver moves up and down as the patient breathes while lying on a treatment table) and/or changes in the shape and size of the target (e.g., a patient's lungs expand, contract and distort during breathing). Finally, there is error due to set up of the patient relative to the radiotherapy machine (e.g., errors in directing the beam toward the patient, etc.). This disclosure is primarily directed toward the second source of error, target motion.
One method for dealing with error due to target motion is to increase the treatment volume (the difference between the treatment volume and a diseased area for which radiation treatment is desired is sometimes referred to as margin) so that the treatment volume is large enough to include the entire range of motion through which the target will travel during the radiation treatment. This approach, however, results in radiation being delivered to normal tissue inside the treatment volume (e.g., normal tissue will be present at the “bottom” of the treatment volume when the target is at the “top” of the treatment volume) and is therefore not desirable.
Another method uses a device to hold a patient's breathing, or requires the patient to hold their breath when the radiation beam is turned on. However, the duty cycles of such systems are low. “Duty cycle” is defined as the percentage of time the beam is on from start to end of each beam delivery, where 100% duty cycle implies no beam interruptions. This scheme also leads to patient discomfort. Moreover, such a scheme is not feasible for patients with certain illnesses such as lung cancer, or patients otherwise having compromised lung functions. This technique is also time consuming in that the period of time for which a patient can hold his or her breath is limited and the delivery of radiation is halted when the patient does not hold his or her breath. Moreover, this technique requires the patient to hold his or her breath at a desired point, and the patient may not be able to hold his or her breath at the desired point accurately.
Another method that has been utilized to reduce error due to target motion is “gating.” In this method, the therapy beam aperture is reduced, to focus on a discrete, pre-selected position of the target at a particular time window during the patient's breathing cycle. It is assumed that the target motion correlates with the breathing cycle, such that the tumor will return to the treatment position at the same time window of the breathing cycle, during the entire course of therapy. A sensor monitors a patient's breath or abdominal excursion (during breathing) and triggers the delivery of a pulse of radiation at the pre-selected time window. The time window may be selected when the patient's lungs are nearly full as the patient inhales (or, alternatively, when the patient's lungs are nearly empty as the patient exhales). This technique is less than optimal as it is time consuming because the radiation is delivered during only a portion (typically 30%-40% time window) of a patient's breath cycle (the duty cycle of such systems is low). In the gating method, the tumor also moves within the time window, although much less than its full range of motion. Therefore, a margin, or a residual margin, is also required to ensure that the tumor gets the intended dose of radiation.
A fourth method that has been explored for reducing error due to target motion is target tracking. In target tracking techniques, the radiation beam follows motion of the target. Three different tracking techniques are presently proposed or implemented. The first tracking technique is achieved by moving the entire treatment head to track the tumor motion, using a robotic arm that carries the accelerator around the patient during the radiation beam delivery. This technique can adjust to changes in patient breathing patterns, but it is impossible for non-robotic, commercially available linear accelerator systems, to emulate such method. Notably, only a small fraction of all linear accelerators or external beam radiation equipment has such robotic capability, and widespread use is expected to be limited. A second tumor tracking technique uses a stationary accelerator equipped with a multi-leaf collimator (MLC), which moves the radiation beam dynamically to track target motions in real time. Dynamically tracking the target minimizes the effects of intra-treatment organ motion, and thereby reduces the margin typically assigned around a moving tumor. Unlike gating, the treatment delivery is not interrupted, and thus is intended to maintain high treatment efficiency. Tracking the tumor with MLC motion also has the advantage of distributing the normal tissue dose over a greater volume, thus lowering the dose burden to the skin and underlying structures. Previous techniques proposed to control the MLC motion to track the tumor along with a program are (1) motion-adaptive x-ray therapy (MAX-T), (2) synchronized moving-aperture radiation therapy (SMART), and (3) aperture maneuver with compelled breath (AMC). Because the motion caused by breathing is more or less cyclical, one can program the MLC to move repeating a pre-defined cycle, assuming that the frequency and amplitude of the motion are correct. But in fact, the motion is not truly following a fixed cycle, instead, there are significant variations in frequency and amplitude between patients and for the same patient at different times (particularly in the high-stress situation that pertains when he or she is undergoing treatment). Accurate tracking of tumor motion with programmed MLC motion sequences requires that the patient's breathing pattern be perfectly consistent with that used for planning. It is well known that patients do not breathe consistently and reproducibly. Significant changes in breathing patterns have been observed within a single breathing session and between different breathing sessions. These irregularities in patient breathing have limited the efficiency of these techniques, leading to the suggestion of combining aperture motion with some kind of patient breath control, thus introducing additional difficulties. Neither MAX-T nor SMART includes an effective method to compensate for irregular breathing, and AMC relies on the patient's active cooperation. MAX-T requires constantly detecting the target position, predicting where it is going, and directing the MLC or treatment table to move; SMART requires the patient to follow a breathing cycle exactly as seen during planning and thus SMART and AMC cannot function in the presence of involuntary patient bodily, or breathing motion. Thus, MAX-T requires real-time feedback control, and for SMART and AMC it is technically challenging to track a tumor solely based on the patient's cooperation, and thus these techniques are further impractical. In order to deliver SMART or AMC, audio and/or video instructions are available to guide the patient to breathe in a consistent pattern. However, such approach has limited applicability, because most patients, especially lung cancer patients, are incapable of breathing without many irregularities; even when they attempt to do so, they most often fail to follow audio or video breathing guidance cues. Although such a pitfall is not present in MAX-T, MAX-T requires a modification of leaf positions in real time, a feature that is not feasible with commercially available MLC systems. In addition to the technical difficulties, real-time modification or creation of a treatment sequence can also raise issues of reliability and safety (for example, the chance for error in calculations performed in real time increases). As a result, no dynamic tracking has been implemented clinically.
In summary, a target tracking method may be used for reducing error due to target motion. In target tracking techniques, the radiation beam follows the motion of the target. However, target motion cannot be accurately predicted. One tracking technique utilizes tracking with guided breathing that requires a patient to follow a breathing pattern to match motion of a radiation beam that is preprogrammed according to the guided breathing pattern. This scheme has problems in that there are some patients that may be incapable of following audio or video breathing guidance cues in spite of extensive training. In another proposed tracking technique, target motion is tracked in real-time and the radiation beam is moved in real time in accordance with the detected motion. This technique is difficult to implement, as the calculations necessary to control movement of the beam are difficult to perform in real time and the MLCs or patient supporting assembly, or table, cannot reach the commanded location quickly. This technique also presents a safety issue as the chance for error in calculations performed in real time increases. Thus, there is a need for a safe and effective method of reducing error due to target motion that circumvents the technical challenges in MLC-based tumor tracking